Use concrete application types instead of associated trait types

[sveitser]

We currently mix the two which may make things harder to read than necessary.

Example (the type signature of metadata) https://github.com/EspressoSystems/espresso-sequencer/blob/2e98bccaceef0334d7ed5253142d8d9c26817263/sequencer/src/header.rs#L246-L259